1. Field of the Invention
The present invention relates to a device for preventing an anchoring element protruding from a distal end of an implantable electrical conductor from coming into contact with and damaging the wall of a body cavity during the conductor's advancement into that cavity.
2. Description of the Prior Art
An implantable electrical conductor, i.e., an electrode cable, is typically designed for connection at its proximal end to a medical device, such as a pacemaker, and its other, distal end is to be affixed to a site in a ventricle or atrium of the heart. Such a conductor can, following intravenous advancement into the heart, be passively or actively affixed to the heart wall to transmit electrical impulses to heart muscle, thereby stimulating the muscle to contract or sense the heart's intrinsic activity as a means of controlling the pacemaker's operation. In "passive" fixation of the distal end of the electrode cable, the end is usually equipped with small fins or tines to facilitate anchoring of the end in the heart wall without penetrating the heart wall.
"Active" fixation of a cable end to the atrial wall is preferred for achieving rapid, forced fixation of the distal end of the electrode cable to the heart wall in the implantation of electrode cable, e.g. when the end of the cable is to be J-shaped and press against an atrial wall. The end of the electrode is then equipped with an anchoring means which can be driven into wall tissue during implantation. The procedure can be monitored by fluoroscopy to ensure that the cable end reaches the correct position.
One anchoring means commonly used for this purpose is in the form of a helical screw with a pointed tip. Electrode ends of this kind require active rotation of the anchoring means in order to screw the tip and a portion of the helical screw into the heart wall. In arrangements in which the helical screw can rotate freely at the end of the electrode cable, the helical screw can be rotated with the aid of a stylet, inserted from the proximal end of the electrode cable and made to grip a polygonal recess at the proximal end of the helical screw (or its mount), and screwed into and anchored in heart wall, the end of the electrode pressing against the heart wall.
In order to keep the pointed tip of the helical screw from coming into contact with and damaging the walls of veins and the heart during the electrode cable's implantation, the aforementioned freely rotating helical screw can be initially retracted into a protective recess at the end of the electrode cable and not deployed, with the aid of the stylet, until it reaches the intended anchoring site just before the helical screw is screwed in.
In another type of anchoring means for active fixation of the end of the electrode in the heart wall, the helical screw is rigidly mounted on the distal end of the electrode and protrudes freely from same. In this instance, the helical screw must be provided with some form of protection to keep the tip of the helical means from damaging venous and heart walls during implantation of the electrode cable. The entire electrode cable, including its sheath, must then be rotated to drive the helical screw into the heart wall.
Different ways of preventing such rigidly mounted, protruding helical screw elements from causing damage during cable introduction have been previously proposed. For example, U.S. Pat. No. 4,827,940 discloses a body, covering the helical screw element, which dissolves in body fluid within a few minutes, whereupon the helical screw can be screwed into the heart wall.
Another proposal is disclosed in U.S. Pat. No. 3,974,834, in which a compressible, bellows-like sleeve on the distal end of the electrode cable protects the helical screw during intravenous advancement of the electrode cable, and is compressed when pressed against the heart wall, enabling the tip of the helical means to gain a footing for screwing into the heart wall.
German OS 3 300 050 shows a version of active electrode fixation (FIG. 4) in which a rounded, tensioned electrode body can be kept extended out of an enclosing helical screw, preventing the tip of the helical screw from damaging the venous wall during intravenous advancement of the electrode cable into the heart.
Furthermore, U.S. Pat. No. 5,261,417 discloses a cardiac pacemaker lead having a protection jacket attached to the tip and a portion of the adjacent convolution of a helix fixing element to prevent damage to the tissue during advancement of the lead through blood vessels and into the heart. The jacket is retractable by a length of wire or cord, either along the external surface of the sheath or through an axial passageway of the conductor after the implantation. Either way, due to the attachment of the jacket to the proper tip portion of the helix element, the jacket may be susceptible to unintentional loosening from the tip portion during the advancement of the lead through the blood vessels.